This invention generally relates to a process for manufacturing printed circuit boards as well as to printed circuit boards manufactured by the process of the invention, and including mounted electronic devices with ease of inspection and repair as well as efficient dissipation of heat.
In the past, heat generated by electronic and electric components is dissipated by employing one or a combination of conduction, radiation and convection techniques. For example, cold plates that depend on the phenomenon whereby electric current is used to convey heat from a cold junction to a hot junction, e.g., Peltier effect, heat pipes that depend on the phase change of material contained in capillary type pipes or comparatively large metal bodies or structures are used to conduct heat from electric and electronic components. The aforementioned heat conduction devices may be used as part of the mounting means for components or may be located adjacent to components. To facilitate heat radiation, black metal bodies with heat radiating fins either are mounted on components or are part of the component mounting means.
To promote thermal convection, forced air moving means and the employ of fan or blower that is either an integral part of an assembly or external to an assembly with or without means for directing and/or controlling air flow are frequently employed. In addition, cooling fluids are at times used for removing heat from components and their assemblies. However, all these techniques add size, cost and complexity to circuit board assemblies and to the higher level assemblies to which they belong. Moreover, these techniques often complicate and add time to the repair and maintenance.
Under current surface mount techniques, as in the case of a pin-grid array microcircuits, soldered connections are made inaccessible for both direct inspection and test. In addition, almost all surface mount techniques cannot employ soldering equipment and techniques, for example, wave soldering, that are generally used for printed circuit boards. Instead, solder preforms, vapor reflow or other special techniques and equipment are required. The consequences are rework or repair of printed circuit board assemblies which is often impossible without specialized equipment and training. Moreover, unless the thermal coefficients of expansion are properly managed and engineered, the solder connections between the microcircuits and the printed circuit board upon which they are mounted are subject to large shearing stresses. These stresses themselves can cause the electrical connections between the micro circuits and the printed circuit board to fail or to have questionable reliability. Examples of complicated circuit board devices carrying chips and having complicated arrangements for mounting and heat dissipation are disclosed in U.S. Pat. Nos. 3,777,221, 3,918,148, 4,197,633 and 4,630,172.
Examples of devices having large scale complex integrated circuits with functional interconnections are disclosed in U.S. Pat. No. 3,795,975 and U.S. Pat. No. 4,640,010.
More recently, in U.S. Ser. No. 2,545 which was filed Jan. 12, 1987, and which disclosure is specifically incorporated by refefence herein, there is disclosed a high resolution circuit technology process for manufacturing high circuit density, copper/polyimide type, multilayer, surface mount, printed circuit boards. In the process as disclosed in the application, the process is used in conjunction with a metal substrate to produce a high circuit density printed circuit board supported by a metal substrate capable of dissipating large amounts of heat. In the basic process as disclosed the printed circuit board is generally manufactured by first applying a first layer of a radiation curable dielectric material to a substrate. A photomask is thereafter placed to define the conductor circuit patterns adjacent to the surface of the radiation curable dielectric material with the material then being exposed to a source of radiation and developed to expose those regions of the substrate where a first conductor circuit pattern is to be formed. Thereafter, the conductor circuit pattern is formed on the surface of the substrate by plating a metal coating onto those regions of the substrate covered by the exposed layer of the first layer of the curable dielectric material to provide a printed circuit board of which the first layer of the dielectric material is a structural component. The details of manufacturing circuit boards in accordance with that method is well disclosed in said copending application Ser. No. 2,545 and will not be discussed in greater detail herein. However, reference is made to that application for details not discussed and for conventional modifications of that technique which will be well known to one of ordinary skill in the art for practicing the present invention.
In accordance with the present invention the method of said copending application is utilized in conjunction with a metal substrate to produce a high circuit density printed circuit board supported by a metal substrate capable of dissipating large amounts of heat. The photoimageable polyimide dielectric is processed in such a fashion that wells for active components such as integrated circuits, i.e., ICs, are created as the circuitry is being laid down through the discussed process. The resulting package will have superior performance characteristics.
More specifically, some of the advantages derived by using the process of the copending application to produce circuitry with wells includes the fact that ICs can be placed in direct contact with the metal substrate to thereby facilitate heat dissipation. Further, the thermal coefficient of expansion of the laminated metal substrate can be tailored to match that of the device. Yet still further, the process disclosed in the copending application is compatible with conventional plated through-hole technology with minimal capital investment so that a switch over to the disclosed process of the copending application can be realized in a conventional printed circuit board shop.
In addition to the above advantages, fine line circuitry, e.g., approximately one mil, and buried coaxial lines which can be improved by using the polyimide process will greatly increase the IC packaging density and wave propagation speeds. Integrated circuits can be connected to circuitry on the printed circuit board using either tape automated bonding, including bare passivated devices, wire bonding, conventional chip carriers and chips that have leads such as gull wing or inverted "J" leads. When conventional chip carriers are used the leads can be soldered to the printed circuit board using conventional soldering methods such as flow solder.
Yet still further, since the ICs are mounted in what may be considered an inverted position, all connections between the ICs and the circuit board are observable and repairable. Visual inspection is thus facilitated and test access is provided which is not provided by current surface mount methods. The repair access feature enables the ICs to be disconnected using conventional tools and techniques and replacement can be also easily accomplished. Yet still further, by mounting the ICs in the wells or pockets, the pockets provide means for minimizing stresses and consequential strains that are induced by a high shock and vibration environment.